OF THE ACALEPH^E OF NORTH AMERICA. 263 



hardly could be visihlo tliiougli the opaque walls; but I am dis])oscd to admit that such a 

 backward current discharges, from time to time, the refuse materials into the main cavity. 

 Indeed, undulatory movements of the globules (Plate II. Fig. 20, b, b) suspended in 

 the liquid are frequently seen in the upper part of the tubes, and the contents of the 

 main cavity hardly leave any doubt respecting the fact, that the undigested materials 

 return to this cavity to be discharged through the mouth. If, upon these facts, we at- 

 tempt to form a precise idea of the nature of the process of digestion and circulation of 

 the nutritive fluid, we remain satisfied that the food introduced into the stomach is there 

 transformed into chyme, under the influence of the epithelial coating and of the secretion 

 issuing from the brown cells which line that cavity and form the inner layer of its tissue ; 

 and that when transformed into a homogeneous mass, or into chyme, it is discharged into 

 the radiating tubes, and circulated through the body. 



The fluid thus circulated and used as nourishment, and assimilated by the parts to 

 restore their deficiency, to increase their tissue, and to contribute to the growth of the 

 animal, is not blood ; nor is it crude food ; it is chyme, properly speaking, but chyme 

 which is circulated, like blood, through a regular system of vessels. But these vessels 

 cannot be compared to blood-vessels, nor this circulation to a regular blood-circulation ; 

 for the tubes communicate directly with the main digestive cavity, and the fluid circulated 

 discharges its refuse parts back into that cavity ; so that we have here a circulation sui 

 generis, and a nutritive fluid also sui generis, which can only be compared to chyme. 

 For if we take the most comprehensive view of circulation, and include under this 

 function the distribution of nourishing fluids of every kind, then we must say that 

 the lowest stage of development of the blood, the lowest condition in which that 

 fluid is circulated, is that which answers to the chyme of higher animals ; and this 

 view will be justified, however strange it may at first appear, by a further comparison 

 of the fluids circulated in the vessels of other animals, if we reflect that, in Articulata 

 and Mollusca, even in those which have a heart, the fluid circulated is not blood 

 properly, in the same sense as the blood of Vertebrata, but chyle. If these views 

 are correct, we should be justified in saying that, in Radiata, chyme is circulated ; in 

 Rlollusca and Articulata, chyle ; and in Vertebrata only, true blood. This view is further 

 sustained by the microscopic examination of the circulating fluid, which consists of glob- 

 ules most heterogeneous, both in respect to their size and their intimate nature, being 

 merely comminuted particles of the digested food. 



As for the tubes themselves, they form, for the circulation of the chyme, a closed sys- 

 tem, which consists of four tubes arising from the upper corners of the central digestive 

 cavity, and imiting at the lower margin with another tube running round that margin. 



